1. Field of the Invention.
The invention described and claimed herein is generally related to methods for fracturing geological formations surrounding well bores, generally for the purpose of stimulating the production of oil or gas.
2. Description of the Related Art.
Various methods have been employed to fracture oil- or gas-bearing geological formations in the vicinity of well bores, for the purpose of stimulating the flow of oil or gas into the well bore. These methods largely fall into two categories; explosive fracturing and hydraulic fracturing.
Explosive fracturing consists simply of detonating a high explosive in a well bore, thereby fracturing the surrounding geological formation. Explosive fracturing was widely used in the oil and gas industry until about 1950, primarily in uncased wells, but has been used only sparsely since that time. It was recognized in the art that conventional explosive fracturing requires large amounts of explosives and, more importantly, results in so much compaction or other damage to the well bore and the surrounding formation as to commonly require extensive clean-up operations after the shot. As a consequence of this relative inefficiency of explosive fracturing, and also as a consequence of the safety hazards associated with handling large amounts of explosives, explosive fracturing has been essentially abandoned and supplanted by hydraulic fracturing.
In conventional hydraulic fracturing, a well bore is filled with a fracturing fluid and is hydraulically pressurized until the formation is fractured, or until "formation breakdown" is obtained. Hydraulic fracturing is typically performed in cased wells. The metal casing is first cemented in, after which both the casing and the surrounding cement are perforated with special explosive perforation charges. The fracturing fluid is then injected under pressure through the perforations and into the formation until the formation fractures, without causing any damage to the casing. Hydraulic fracturing is typically augmented by the use of proppants and other well known oil recovery stimulation methods.
Although hydraulic fracturing has been widely used, it nevertheless suffers from several problems. One problem is that very high pressures may be required, particularly in deep wells. Occasionally the pressure required to fracture deep formations is so high as to be beyond the capacity of commercially available hydraulic fracturing equipment.
Perhaps the most significant problem, however, is that hydraulic fracturing typically produces only a single fracture, or at most a very small number of fractures. This occurs because it requires considerably less pressure to enlarge an existing fracture than to create a new one. Consequently, once a first fracture has been created, further pumping simply results in the first fracture being further propagated. Additional injecting of fracturing fluid simply results in the fluid being injected into the first fracture, with no additional fractures being created. It is typically impossible to create additional fractures once the formation has been initially fractured, and thus the stimulation of oil or gas flow is limited to that which can be obtained by the creation of a single fracture.
Furthermore, the location at which fracturing occurs is determined by the weakest point in the formation. It is commonly found that one or more oil- or gas-bearing formations occur only at particular depth ranges within a well, and hence it is desired to hydraulically fracture only those formations. Because of the difficulty noted above in forming more than one fracture, packers, high pressure tubing and other special equipment are used to localize the region of the well bore that is pressurized and fractured. Although these devices permit some control over the depth range at which a fracture is formed, they are costly and time consuming to install and utilize effectively.
Another limitation of hydraulic fracturing is that there is no way to control the direction of fracturing. In some cases it is desirable to be able to selectively fracture a formation in a particular direction away from a well bore. The packers and other equipment mentioned above provide some control over the depth at which fracturing occurs, but provide no control over the direction of fracturing away from the well bore.
U.S. Pat. No. 2,676,662 to Ritzmann discloses a method for enhancing oil recovery in a well. In accordance with the method of Ritzmann, a formation is first hydraulically fractured and then subjected to an explosive detonation to created irregularities in the hyraulically-formed fractures and thereby prop the fractures open. The method of Ritzmann differs from the method of the present invention in that, as will be discussed below, in the method of the present invention the formation is not hydraulically fractured prior to detonation.
Accordingly, it is the object and purpose of the present invention to provide an improved method of fracturing a geological formation surrounding a well bore.
More particularly, it is an object and purpose of the present invention to provide a method of forming a large number of radially extending fractures in a geological formation surrounding a well bore, without in the process significantly damaging the well casing or compacting the adjacent geological formation.
It is another object and purpose of the present invention to provide a method of fracturing a geological formation, in which both the depth and the direction of fracturing can be easily controlled.